1. Field of the Invention
The invention relates to a device for the precise or register-true chucking of form plates, particularly offset printing plates, onto plate cylinders of printing presses.
2. The Prior Art
In offset printing processes, such a multi-color offset printing, it is highly desirable to register the offset plates on the plate cylinders of the individual printing units in such a manner that no contour variations, sometimes called register errors, occur in the printing among the individual colors. Devices for adjusting the register axially of the cylinder and at right angles thereto are utilized in plate cylinder machines. Adjusting the register before printing begins, however, is time-consuming. With respect to proofing presses, there is a need to quickly print a specimen and then again release the machine, such that register time and printing time both need to be kept to a minimum. Known register adjustment means can usually only adjust the register in the two aforementioned directions. However if a register error occurs, such as may occur when plates are applied to the cylinder twisted in the manner of a helical line which may produce oppositely facing plate inclines between individual colors in a multi-color offset printing process, then correction by means of the register adjustment device is no longer possible. In order to produce a precise and faultless impression, the printing plates must be re-chucked which wastes a great deal of time. The situation is even more cumbersome when a so-called inner register error is produced as a result of the chucking device or faulty manipulation. Inner register error occurs when the printing plate has suffered deformations over such an area that register-true printing cannot occur. When such deformations are permanent, the plate is lost and must be recopied.
One type of known plate chucking device requires that the plate, prior to application to the plate cylinder, be rounded or specially pre-shaped in some manner at its contour edges by means of separate devices, such as disclosed in German AS 2744371 and German AS 2116570. Printing plate spacing devices require complex structure and their operation is prone to errors and is time-consuming. Furthermore, pre-shaping can rarely be executed with register precision. However, pre-shaping of plate edges is effective in orienting the plate in the chucking means of a plate cylinder. In some instances, alignment pins are formed on the cylinder and corresponding register perforations made in the plate to further ensure plate positioning on the cylinder; but these devices are merely redundant.
In another form of chucking means, printing plates are held by means of blocks positioned on the cylinder beneath clamping beams, such that a forward edge of the printing plate is frictionally held between the underside of a clamping beam and a corresponding flat surface of a block. Clamping pressure is typically produced by means of screws axially-spaced along the cylinder for correspondence with the clamping beams. The clamping beam may occur as a single element or a set of plural such devices and beams may be present for axial displacement of the clamping beam within certain limits in order to compensate for chucking errors. In some versions, register pins and plate perforations are also utilized for aiding in the alignment of the forward edge of the printing plate on the cylinder. However, due to the relative thinness of offset plates, the pin perforations can be stripped as printing pressures warp the plate. Accordingly, alignment produced with pins and perforations is imprecise and adjustment is time-consuming.
A drawback common to all plate chucking devices utilizing a clamping force is that the frictional force required for clamping a plate must be very great in order to counter tangential tension along the cylinder. In large-format presses, such tangential tension can range in the magnitude of 1,000 kg and, even if reduced by frictional resistance, is still a couple hundred kg at the clamping beam. When such forces are applied at the clamping beam, such as by screws, then one comes very close to that pressure which would damage or even destroy the plate. Torque-limiting tools for tightening clamping beam screws can only incompletely reduce the risk of damaging the plates in this manner. The conversion of torque into a pressing power greatly depends on the condition of screw threads, lubrication, etc. In practice, tolerances occurring in the screw means are usually greater than the narrow tolerance required for the requisite pressing power. Attempts have been made to increase the friction at the clamping beam by means of corresponding engagement surfaces in the beam and cylinder block, for example, interengaging undulating or triangular tooth-shaped ridges and grooves. However, this solution requires a greater surface area of the offset plate to be taken up in the chucking area, because of the permanent deformation such gripping arrangements necessitate. In addition, register observation or even a pre-alignment by the use of pins and perforations is not possible with adequate precision.
The present invention is drawn to a chucking device by which the aforementioned problems are reduced and which lends itself to short setup times. A chucking device which places the plate on the cylinder quickly and enables true register alignment independent of the skill of an operator is much needed in the art.